Converters may be used to transform voltages. For example, a buck converter may be used to transform a direct current (DC) input voltage into another direct current (DC) output voltage. The output voltage may be smaller than the input voltage. The average value of the output voltage may be adjusted by setting periodic opening and closing times of a switch. When the switch is closed, a current builds up in an inductor and current flows through a load. When the switch is opened, a freewheeling device allows the current in the inductor to continue to flow and the load is supplied with energy stored in the inductor.
Protection against electric shock may be achieved by using Extra Low Voltages (ELV) or Safety Extra Low Voltage (SELV) in electrical circuits. Protective separation, for example in the form of double isolation, reinforced isolation or protective screening, may also be used for this purpose. If the electric potential of any conductor, including those of control signals, against ground is not larger than 25 V RMS (35 volts peak) for alternating current or 60 V for direct current and the leakage current is not larger than 2 mA DC, protective separation may not be needed.
A N-channel MOSFET may be used without complex driving as a low side switch in a buck converter. In this case, the load is usually connected to the positive potential of the input voltage while any communication interfaces (for example temperature sensing) remain at MOSFET source potential. Therefore, if the converter output is to fulfill SELV requirements without protective separation, the maximum allowable input voltage is limited to the maximum SELV voltage (60 V max). The converter output voltage available to the load will then typically be about 5 to 50 V.
The converter output voltage limitations can be overcame by using a high side switch. However, driving a high side switch is more complicated and will lead to higher costs for components.
It is desirable to provide a low-cost, SELV compliant converter with an output voltage of up to 60 V.